Magnetometry

Satellite magnetometry measures Earth's magnetic field vector and scalar magnitude from low Earth orbit using high-precision magnetometers, mapping contributions from the geodynamo core field, lithospheric magnetisation, and external ionospheric and magnetospheric current systems.

Two instrument types complement each other on modern missions. Fluxgate vector field magnetometers (VFMs) measure three orthogonal field components at high cadence; accurate vector measurements require precise attitude quaternions from co-aligned star trackers to rotate observations to the geographic frame. Absolute scalar magnetometers (helium resonance or Overhauser proton devices) measure field magnitude to sub-nanotesla accuracy and serve as the in-flight calibration reference for the VFM, maintaining stable vector accuracy throughout the mission lifetime. The global geomagnetic field intensity spans approximately 25,000-65,000 nT depending on latitude.

Observed magnetic field measurements are inverted iteratively using spherical harmonic forward models (the CHAOS series, the International Geomagnetic Reference Field, IGRF) that separate internal contributions - core dynamo and lithospheric remanent magnetisation - from external contributions driven by ionospheric and magnetospheric current systems. Flying two satellites approximately 15 km apart at the equator (magnetic gradiometry, as implemented by Swarm A and C) improves spatial separation of core and lithospheric source fields.

The dominant operational mission is ESA's Swarm constellation (Swarm A, B, and C, launched November 2013): Swarm A and C fly side-by-side at approximately 462 km altitude and 87.35 degree inclination; Swarm B flies a higher orbit at approximately 511 km and 87.75 degree inclination for temporal de-aliasing of external field signals. Earlier missions include Oersted (Denmark/DTU, 1999-2013, the first modern low-Earth orbit scalar and vector magnetometry mission), CHAMP (DLR, 2000-2010, combined magnetometry and gravimetry), and SAC-C (Argentina/NASA, 2000-2013).

Applications include core field monitoring and secular variation analysis for geodynamo research and IGRF model updates, lithospheric magnetisation and crustal geology mapping, three-dimensional mantle and crustal electrical conductivity determination, ionospheric and magnetospheric current system characterisation (ring current, field-aligned currents), space weather monitoring and geomagnetic storm index derivation, and detection of ocean flow magnetic induction signatures from saline water motion.

The long multi-mission baseline from Oersted (1999) to the Swarm constellation enables climate-scale secular variation analysis. Key limitations include contamination of core and lithospheric signals by dynamic external fields requiring geomagnetically quiet-time data selection, lateral resolution limited by satellite altitude (approximately 50-100 km for lithospheric anomalies), and the need for airborne or ground surveys to resolve short-wavelength crustal detail below satellite resolution.